Deprecate user data plugins

[ahmedwaleedmalik]

User data plugins from machine-controller have been deprecated and will soon be removed in favor of OSM.

What this PR does / why we need it:

Historically, machine-controller relied on user data plugins to generate configurations(cloud-config) for worker machines. Each operating system requires its own user-data plugin. These configs are then injected into the worker nodes using provisioning utilities such as cloud-init or ignition. Eventually, the nodes are bootstrapped.

Over time, it has been observed that this workflow has certain limitations.

• Machine Controller expects ALL the supported user-data plugins to exist and be ready. User might only be interested in a subset of the available operating systems. For example, user might only want to work with ubuntu.
• The user-data plugins have templates defined in-code. Which is not ideal since code changes are required to update those templates. Then those code changes need to become a part of the subsequent releases for machine-controller and KubeOne. So we need a complete release cycle to ship those changes to customers.
• Managing configs for multiple cloud providers, OS flavors and OS versions, adds a lot of complexity and redundancy in machine-controller.
• Since the templates are defined in-code, there is no way for an end user to customize them to suit their use-cases.
• Each cloud provider sets some sort of limits for the size of user-data, machine won't be created in case of non-compliance. For example, at the time of writing this, AWS has set a hard limit of 16KB.
• Better support for air-gapped environments is required.

To overcome these limitations; Kubermatic built Operating System Manager. Please refer to Operating System Manager for more details.

User data plugins from machine-controller have thus been deprecated and will soon be removed in favor of OSM.

Which issue(s) this PR fixes:

Fixes #1544

What type of PR is this?

/kind deprecation

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

[ACTION REQUIRED] User data plugins for machine-controller are deprecated and will soon be removed. [Operating System Manager](https://github.com/kubermatic/operating-system-manager) is the successor of user data plugins. It's responsible for creating and managing the required configurations for worker nodes in a Kubernetes cluster with better modularity and extensibility. Please refer to [Operating System Manager](https://github.com/kubermatic/operating-system-manager#kubermatic-operating-system-manager) for more details.

Documentation:

https://docs.kubermatic.com/operatingsystemmanager/

[xrstf]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 54af6e282bb61515d1983c059eee85762bcabb80

[kron4eg]

/approve
/lgtm

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmedwaleedmalik, kron4eg

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[ahmedwaleedmalik]

/retest

[ahmedwaleedmalik]

/retest